Particle accelerators are employed to irradiate a wide variety of materials for several purposes. One purpose is to facilitate or aid molecular crosslinking or polymerization of plastic and/or resin materials. Other uses include sterilization of foodstuffs and medical supplies and sewage, and the destruction of toxic or polluting organic materials from water, sediments and soil.
A particle beam accelerator typically includes (i) an emitter for emitting the particle beam, (ii) an accelerator for shaping the emitted particles into a beam and for directing and accelerating the highly energized particle beam toward a transmission window, (iii) usually a beam scanning or deflection means and (iv) a transmission window and window mounting. A generator is provided for generating the considerable voltage difference needed to power the accelerator.
The emitter and the accelerator section, which may comprise centrally arranged dynode elements or other beam shaping means, or electrostatic or electromagnetic lenses for shaping, focusing and directing the beam, are included within a highly evacuated vacuum chamber from which air molecules have been removed so that they cannot interfere with the particle beam during the emitting, shaping, directing and accelerating processes.
The term "particle accelerator" includes accelerators for charged particles including, for example, electrons and heavier atomic particles, such as mesons or protons or other ions. These particles may be neutralized subsequent to acceleration, usually prior to exiting the vacuum chamber.
The transmission window is provided at a target end of the vacuum chamber and enables the beam to pass therethrough and thereby exit the vacuum chamber. The workpiece to be irradiated by the particle beam is usually positioned outside the accelerator vacuum chamber and adjacent to the transmission window in the path of the particle beam.
As used herein, "transmission window" is a sheet of material which is substantially transparent to the particle beam impinging thereon and passing therethrough. The transmission window is mounted on a window mounting comprising a support frame which includes securing and retention means which define a window envelope.
The conventional beam transmission window, usually rectangular with filleted corners and generally perpendicular with respect to a longitudinal axis of the particle beam, must be sufficiently thin and of a suitable material so as not to attenuate the beam unduly from energy absorption and consequent heating. The window material must be sufficiently strong to withstand the combined stresses due to the pressure difference from typical ambient atmospheric pressure on one side thereof and high vacuum on the other and due to the heat generated by the particle beam in passing therethrough.
Conventionally, transmission window foils have typically been lo installed between rectangular, generally flat flanges with filleted corners. The thin window foils are typically formed of titanium or titanium alloy sheets or foils which typically range in thickness between about 0.0005 inches (0.013 mm) and 0.004 inches (0.104 mm). Much thicker stainless steel foils have been employed as transmission windows in irradiation apparatus for waste water/effluent processing.
When vacuum is drawn on one side of a conventionally installed, flat foil window, the ambient air pressure on the other side tends to deform or "pillow" the foil window slightly. Part of this deformation results from transverse stretching of the foil. The radius of curvature of the foil resulting from drawing a vacuum is defined by the amount of transverse stress incurred. The relation therebetween for a foil of indefinite length (that is, neglecting end effects) is given by the following: EQU S1=(p(R/t) transverse stress (lb/in2)
where
p=differential pressure across foil (lb/in2) PA1 R=radius of curvature (inches) PA1 t=thickness of foil (inches); and PA1 S2=S1.sup.2 axial stress (lbs/in2) PA1 generating a particle beam within a vacuum chamber, PA1 directing the particle beam toward a particle beam transmission window at a radiation emission end of the vacuum chamber, PA1 supplying from a source a quantity of said material to be processed within a fluid medium, such as a liquid, PA1 directing a flow of the fluid medium supplied from the source against an exterior surface of the particle beam transmission window in order to transfer heat therefrom to the medium, PA1 simultaneously exposing the material in the fluid medium to accelerated particles of said particle beam passing through the transmission window means in order to process the material.
and the total stress S at any position on the window is given by: EQU S=(S1.sup.2 +S2.sup.2) (given in lbs/in2).
Because the window is not of indefinite length, the ends thereof are subjected to additional axial stress as well as transverse stress because of the transverse and end retention structure adjacent thereto. The combination of axial and transverse stresses often results in wrinkling, non-uniform deformation, or even actual creasing at the window ends, and increases the chances of premature failure thereat.
Because the sheet or foil materials used for conventional window configurations have inherent strength limitations, particle accelerator power output is limited, not by the high voltage generator capacity, but by the maximum heating due to the particle flux that the window material can withstand. The prior art has therefore sought to minimize the increase in temperature of the window during accelerator operation or decrease the mechanical stress it is subjected to. One known technique includes, for example, providing support grids inside the accelerator chamber and abutting against the window. In this particular technique, the support grids are often cooled by coolant flowing through internal cooling passages. While this technique effectively increases the active window area, the grids used in these known designs are within the beam path and therefore undesirably absorb a significant fraction of the incident accelerated particles. By "active window area" is meant that area of the window within and defined by the securing structure and having an active transverse dimension. A related technique of increasing the window area without providing additional support increases the tendency of the window foil to fail under stress. Thus, a hitherto unsatisfied need has arisen for an improved transmission window design wherein a given thickness of window foil can withstand a much higher particle flux than that contemplated heretofore.
The efficacy of radiation-thermal cracking (RTC) and viscosity reduction of light and heavy petroleum stock, for example, has been reported in the prior art. Also, high energy particle experiments have been conducted in connection with processing of aqueous material including potable water, effluents and waste products in order to reduce chemically or eliminate toxic organic materials, such as PCBs, dioxins, phenols, benzenes, trichloroethylene, tetrachloroethylene, aromatic compounds, etc.
The techniques heretofore employed have typically presented a liquid sheet or "waterfall" in front of, but spaced away from, the particle beam. Conventional wisdom associated with these techniques has been to employ very highly energetic particle beam sources (e.g. 1-3 MeV) in order to obtain sufficient particle penetration. In order to process usefully large quantities, high beam currents, such as 50 milliamperes or more have also been proposed. High energy and high beam currents require very expensive voltage generation and beam forming apparatus.
However, McKeown (Radiation Physics and Chemistry, volume 22, 1983, pp 419-430) in a paper entitled "Electron accelerators--a new approach" has disclosed a waste water irradiation target chamber comprising a curved vacuum window of 0.75 mm thick stainless steel welded to a fiat window surround, apparently of the same material. Waste water to be irradiated passes through a u-shaped structure containing the window in one arm. The impinging scanned electron beam was produced by a microwave accelerator and had an energy of 4 MeV. He states: "The scanned 4 MeV beam penetrated a 0.75 mm thick stainless steel into a fast flowing effluent target to test the design criteria of the mechanical and thermal stresses in the window . . . Experiments showed that sustained power dissipation of 100 W/cm.sup.2 on the window showed no deterioration and failure occurred at 3.5 times this design value."
A power dissipation of 100 W/cm.sup.2 in a window 0.75 mm thick results in a thermal load to the scanned portion of the window of 168 W/g (watts per gram). Failure thus occurred at a window thermal loading of 584 W/g. Energy losses in a 4 MeV beam passing through such a window would exceed 24%, that is, about 33.8 keV per mil of window thickness. Furthermore, on page 423 of this reference it is stated "FIG. 6 is a symbolic representation of the main elements which make up a linac-based accelerator. The efficiencies shown have already been achieved under optimum conditions and it now seems possible that total conversion of main power to electron beam power in the target could exceed 50%." FIG. 6 of this reference shows that the conversion efficiency before the window for a 10 MeV linac is 60.6%. As the window shown in this figure is stated to pass electrons of this energy through with 90% efficiency, the total delivered efficiency would be expected to be 54.53%.
The use of a thin sheet of liquid material being irradiated has not been simultaneously employed to transfer heat away from a curved transmission window of the beam. Heretofore, there has been an unsolved need for a lower particle energy, higher beam current, higher efficiency irradiation apparatus for radiation processing of materials such as petroleum stock, potable water, effluents and other aqueous and liquid materials. We have discovered that, contrary to the teachings of McKeown and the general understanding of the prior art, by the use of a curved transmission window one can greatly reduce the stresses caused by the pressure differential thereacross during operation, thereby enabling use of highly electron transparent window foils in demanding operating conditions. This discovery enables us to provide a highly efficient rugged high power particle accelerator apparatus, which may easily be rendered transportable.